The overall goal of this proposal is to further characterize the neuroendocrine mechanisms that underlie the initiation and expression of daily torpor, an extreme form of temperature regulation that evolved to help animals contend with limited food availability. The relation of torpor to overall energy balance and body fat stores will be explored. Annual rhythms of daily torpor are well documented in the Siberian hamster (Phodopus sungorus), the model species in all experiments. Specific aims include: 1) clarifying the relation between decreases in body fat stores and its feedback mechanisms in the initiation of torpor; 2) characterizing the role of leptin as a metabolic signal inhibiting initiation of torpor; 3) exploring the role of NPY in torpor; 4) determining the role of diencephalic mechanisms in initiation of daily torpor There is much that we still do not understand about the physiological mechanisms underlying the control of energy balance and thermoregulation. Delineating the interaction between metabolic feedback signals and daily torpor in a model system like the Siberian hamster could provide important insights into the regulation of these processes for mammals in general and humans in particular. A better understanding of the mechanisms of reversible hypothermia is of medical import. The reduced metabolism, blood flow, etc. coincident with hypothermia may be very beneficial during many types of major surgery. At this time, we know little of the mechanism that allows Siberian hamsters and several other mammalian species to undergo hypothermia which is lethal to humans and most other mammals. The proposed research addresses fundamental questions in regulatory biology and, thereby, affords the opportunity of establishing general principles applicable to all mammals.